Monday, June 22, 2009

The Old Croton Aqueduct: An Engineering Marvel

LOWER HUDSON VALLEY

Seen from the air, the Old Croton Aqueduct gives the impression that a giant mole had tunneled its way south from the Croton River to New York City, throwing up the slight bulge that is the telltale sign of the animal's passage through a lawn. Now a public right of way, the aqueduct was purchased by New York State from New York City's Bureau of Water Supply in 1968. Listed on both the New York State and the National Registers of Historic Places in 1974, most of it is open for walking and cycling.

This elongated but little-used state park extends all the way to the heart of New York City. In its northern reaches, it closely parallels the Hudson River. Comparatively straight and somewhat narrow as parks go, it sometimes follows city streets and highways; at other times it traverses meadows and wooded areas. It was built with a singleness of purpose: to carry water to a thirsty, disease-ridden, fire-threatened city more than a century and a half ago. In selecting its route, engineers were mindful that the water coursing through it would be impelled only by gravity. The total drop between Croton and Manhattan is 43.63 feet, or 13-1/4 inches per mile.

The aqueduct starts near the New Croton Dam, makes its way to Ossining, crossing Quaker Bridge Road twice. In Crotonville, it traverses the Indian Brook Culvert, which allows the stream of that name to pass under the aqueduct embankment. Before reaching Main Street in Ossining, it crosses the famous Double Arch Aqueduct Bridge. It runs parallel to Main Street and then across Nelson Park. It crosses Route 9 once again to Scarborough Road and back again before passing behind the Old Dutch Church in Sleepy Hollow.

The aqueduct brings walkers close to some of Westchester's architectural gems: In Tarrytown, financier Jay Gould's Gothic Revival mansion, Lyndhurst; in Irvington, Washington Irving's home, "Sunnyside," the Armour-Stiner domed octagonal house, and Nevis, the mansion built by Col. James A. Hamilton and named for the island in the West Indies where his father, Alexander Hamilton, was born; in Hastings-on-Hudson on Aqueduct Lane, the former studio of Russian-born French sculptor Jacques Lipchitz.

More than a few hardy walkers have hiked the entire length of the Old Croton Aqueduct, 26 miles of which are within the present boundaries of Westchester County. Walking the aqueduct is one way to appreciate this engineering marvel of the nineteenth century. Although some of its features are concealed in the earth, many can be discerned by observant walkers.

Building the Aqueduct
Chief Engineer John B. Jervis organized the construction of the aqueduct into four 10-mile-long divisions, each under the supervision of a resident engineer, and 96 subdivisions. called "sections." Jervis's annual salary was $5,000; his assistant, Horatio Allen, earned $3,500 a year. Resident engineers made between $1,500 and $1,800. The pay of the Irish laborers who built the aqueduct ranged between 75 cents and a dollar a day.

The major portion of the aqueduct was constructed by the cut-and-fill method in which a trench was dug, the aqueduct was built, and the earth was backfilled over it. The decision to enclose the aqueduct completely with ramparts of earth was made to protect it from the weather and contamination by ground water.

The following are the specifications of the aqueduct, taken from an official report: "The foundation is stone, upon which is laid a bed of concrete, composed of broken granite and hydraulic cement; the side walls are of hammered stone, laid up with cement; the floor is composed of an inverted arch of hard brick, eight inches thick; the lining of the side walls and the upper roof arch are of the same thickness and materials, all laid with hydraulic lime mortar. No common mortar is permitted in the whole structure."

Hydraulic cement has the advantage that the more it is exposed to water, the harder it gets. Maximum interior dimensions were 8 feet 5-1/2 inches high and 7 feet 8 inches wide--larger than any aqueducts constructed previously in Europe.

The Old Croton Dam
Simultaneous with the construction of the aqueduct, work was begun in January of 1838 on a dam in the valley of the Croton River, a huge earthen embankment about 250 feet long and 65 feet high. It was 250 feet wide at the base, tapering upwards to 55 feet wide at the top. The narrow spillway portion of the dam over which a sheet of water flowed was the only part that was constructed of stone. In times of heavy rain, it was assumed that the outflowing water would measure no more than 4 to 6 feet deep.

After the first season of construction, 2,445 feet of aqueduct had been completed and 635 feet of tunnels had been dug. At the end of the second year, 11.2 miles of the aqueduct had been completed. By January of 1841, about 32 miles, or two-thirds of the aqueduct had been constructed. The Croton Dam was nearly finished, a massive masonry arch bridge had been built at Sing Sing (Ossining) and twelve tunnels with a total length of 4,406 feet had been dug.

Ventilators and Waste Weirs
Thirty-three cylindrical stone ventilator shafts were erected to keep the aqueduct at atmospheric pressure. In the northern part of the aqueduct these were located about a mile apart; some no longer stand. Eleven ventilators had access doors to allow for inspection of the aqueduct. Ventilator No. 8 can be seen near the Park School in Ossining.

To allow water to be drained from the aqueduct if the level rose above a certain height, six waste weirs with hand-controlled gates and waste outlets were constructed. These also provided ventilation and access. A waste weir can be seen just before Snowden Avenue at the northern end of the Double Arch Bridge in Ossining. Contracts were let for the construction of six residences for the keepers of the waste weirs. One of these, built in 1845, survives at Dobbs Ferry, along with its maintenance barn.

Inverted Siphons
As almost everyone knows, water in a U-shaped pipe will ascend to the same height on each side. Such a device is called an inverted siphon. To avoid having to use large embankments or viaduct bridges, an inverted siphon of cast iron pipes three feet in diameter crossed the deep Manhattan Valley at 125th Street in New York City. To increase the flow, the elevation at the exit was three feet lower than at the entrance.

Embankments and Culverts
To avoid crossing wide valleys on bridges or viaducts with the possibility of frost damage, large embankments were built. The aqueduct was buried at the top of these below the frost line. At the bottom, an arched culvert faced with cut stone allowed a stream or road (or both) to pass through. Buttresses and wing walls were built at each end of the culvert to lead water away; even streambeds were lined with stone. Parapet walls were constructed across the tops of culverts to hold back the earthen embankments above them.

In all, 114 culverts were constructed, ranging from spans of 1-1/2 feet to 25 feet, to permit streams or roads to pass through embankments. The total length of all culverts on the aqueduct was 7,959 feet.

Embankments may be viewed where the aqueduct crosses roads, such as Station Road in Irvington, Quarry Road in Hastings-on-Hudson and Nepperhan Avenue in Yonkers. A very large embankment some 1,900 feet in length carried the aqueduct across the broad Clendenning Valley in Manhattan between 95th and 102nd Streets. The latter proved to be such an impediment to crosstown traffic that it was removed in the 1870s and replaced by undergound piping.

Tunnels
Whenever a hill or a ridge lay in the path of the aqueduct, a tunnel was dug. Sixteen tunnels lie along the route of the aqueduct, varying in length from 160 to 1,263 feet, with a total length of 6,841 feet. When tunnels were drilled through solid rock, the roof was not lined. The walls and floor, however, were constructed in the same manner as the aqueduct. This identical profile provided the same flow characteristics throughout the length of the aqueduct as far as the Harlem River.

Tunnels on the aqueduct are located near Quaker Bridge in Croton-on-Hudson, just before the Double Arch Bridge in Ossining, under Cedar Street in Dobbs Ferry, and in Yonkers where the aqueduct turns east to cross into the Saw Mill River Valley.

Meanwhile, Back at the Dam
Friday, January 8, 1841, was a black day for the Croton aqueduct project. A thaw had set in earlier, melting the 18 inches of snow that blanketed the surrounding hills. A heavy rain had also fallen for two days. Behind the nearly finished dam, the water rose at the alarming rate of one foot an hour.

It soon became obvious that the dam's masonry spillway was entirely too small to handle such a volume of water. The reservoir began to pour over the earthen walls that linked the spillway to the north bank of the Croton River. Sensing impending disaster, Alfred Brayton, the young son of one of the dam contractors, heroically rode off to warn residents farther downstream. His errand of mercy was made more difficult because some of the bridges had already been carried away.

Eventually, the water reached a height of fifteen feet above the spillway. At 4:30 in the morning, the earthen embankment failed, unleashing a torrent of water, mud, rocks and debris consisting of uprooted trees, small buildings and the remains of Pines Bridge that had been literally lifted off its footings by the rising waters. Three lives, one at the dam and two at Bailey's wire mill, were lost in the tremendous deluge that washed out every bridge across the Croton River, three mills and five or six houses. The cost to the city for property damage was about $75,000. So much mud and debris was carried downstream, however, that the mouth of the Croton River, formerly navigable by schooners and sloops as far as Underhill's gristmill at Quaker Bridge, became silted up and shallow.

The Rebuilt Old Croton Dam
because the severity of the storm was unprecedented, John B. Jervis managed to retain his job. Loss of the dam was a bitter lesson, but he profited from it. The new dam was redesigned on a radically different plan and rebuilt for $60,000. Constructed this time entirely of stone, with a rubble core and cut granite facing, it had a height of 57 feet; the spillway to carry off the overflowing water stretched almost the full width of the dam.

Jervis was acutely conscious of the scouring power of swiftly rushing water now and designed the masonry face of the new dam so that any overflow would be carried over it safely. In cross-section it was a gentle S-shaped ogee curve that eased the water over the top of the dam and down its face, discharging it horizontally into a backwater pool created by a secondary rock-and-timber cribbed apron, thus slowing it down.

Completed in 1842, the rebuilt Old Croton Dam was the first large masonry dam constructed in the United States and became the model for municipal water system dams in the United States for many years. Its site is three miles to the north of the present Cornell Dam, or New Croton Dam. At periods of extremely low water, the outlines of the intact Old Croton Dam can be seen. Interestingly, the submerged Old Croton Dam and its gatehouse and sluiceway (at its southern end near where the original inlet to the Croton Aqueduct was located) are listed on the National Register of Historic Places as an underwater archeological site.

Aqueduct Bridges
Several bridges were built to carry the aqueduct over existing streets and roads. The most impressive of these is the so-called Double Arch Bridge in Ossining. Spanning the Sing Sing Kill in a north-south direction, the original arch is 88 feet long and 76 feet high, yielding a structure of striking boldness.

In the 1860's, a second arch was added through the larger arch in an east-west direction to carry Broadway across the same watercourse. The twin arches present an arresting architectural composition that is one of the conversation pieces of the Old Croton Aqueduct.

Up until 1924, a massive aqueduct arch to the south of Scarborough spanned the Albany Post Road (Route 9) and gave the hamlet of Archville its name. It was removed by court order and replaced by an unobtrusive underground inverted siphon after the comparatively narrow opening of the arch that spanned the road became a traffic hazard. After an absence of 74 years, in 1998 a rustproof COR-TEN steel footbridge replaced the former arch. Stone embankments that connected to either side of the original arch can still be seen along Route 9 about a quarter-mile north of the intersection with Route 117. This is the only location along the entire length of the aqueduct where the original right-of-way has been modified.

How to get Croton water across the Harlem River became a political football. Some wanted a tunnel under the river, estimated to cost $636,738--but tunneling technology was in its infancy at the time, and the uncertainty of pursuing this option led to its rejection. This left a bridge as the only recourse, with the Water Commission, engineers and the public split between a low bridge and a high bridge. Speculators and lobbyists for area landowners wanted a truly imposing structure. Jervis favored a low bridge, which would have been simpler, faster, and cheaper to construct. The estimate for a high bridge was $836,623. A low bridge would have been cheaper than either of the other two crossings. When concerns were raised in the State Legislature that a low bridge would obstruct boat passage along the Harlem River to the Hudson River, a high bridge was ultimately chosen.

In June of 1839, after two years of haggling, the Water Commission agreed on a high bridge. The contract for the bridge was awarded to a consortium of contractors, Timothy Ferell, Samuel Roberts, Arnold Mason and George Law. Three of these were already contractors on other sections of the aqueduct.

Reminiscent of classical aqueducts, the bridge Javis designed, aided by James Renwick, Jr. (who would later design St. Patrick's Cathedral), was 1,450 feet long with 15 arches 100 feet above the river. Eight of its fifteen arches were anchored in the river itself, with the remaining seven standing on the land. The river arches were 80 feet ide and 150 feet thigh. The seven land arches were fifty feet wide; one was on the steep Manhattan shore and 6 were on the upward-sloping Westchester (now the borough of the Bronx) side of the river.

The river bottom proved to be a mix of mud, hard sand and boulders over the underlying bedrock. To support the graceful arches of the bridge, clusters of oak piles had to be driven into the river bed to support five of the eight piers. The Croton water was carried across the bridge in an inverted siphon of twin 48-inch pipes, covered for protection from the elements by five feet of earth. To increase pressure and flow, the exit on the Manhattan side was placed two feet lower than the entrance.

After its completion in 1848, the High Bridge and its picturesque arches quickly became a popular romantic subject for paintings by artists of the Hudson River School. Unfortunately, the bridge lost much of its classical look in the twentieth century when five of the arches resting on the river bed were removed and replaced by a soaring steel span that offered no impediment to busy ship traffic. The opening ceremonies for the new bridge were held on Oct. 27, 1928.

Today, the grand old aqueduct itself remains remarkably unchanged, a unique legacy somnolently waiting for a later generation to discover it and explore it. And the best way to do this is afoot. Readers interested in walking the aqueduct will find useful a large-format pamphlet entitled A Walker's Guide to the Old Croton Aqueduct, published by and obtainable from the N.Y. State Office of Parks, Recreation and Historic Preservation, Taconic Region, Staatsburg, NY 12580 (914/889-4100). It is also available in most Westchester public libraries. A free trail map can be obtained by sending a stamped and self-addressed envelope to Old Croton Trailway State Park, 15 Walnut Street, Dobbs Ferry, NY 10522 (914/693-5259).